This invention relates generally to wireless communications circuitry, and more particularly, to wireless communications circuitry that controls radio-frequency output powers in electronic devices using closed-loop and open-loop control.
Handheld electronic devices and other portable electronic devices are becoming increasingly popular. Examples of handheld devices include handheld computers, cellular telephones, and media players. Popular portable electronic devices that are somewhat larger than traditional handheld electronic devices include laptop computers and tablet computers.
Due in part to their mobile nature, portable electronic devices are often provided with wireless communications capabilities. For example, portable electronic devices may use long-range wireless communications to communicate with wireless base stations. In particular, cellular telephones and other devices with cellular capabilities may communicate using cellular telephone bands at 850 MHz, 900 MHz, 1800 MHz, and 1900 MHz. Communications are also possible in the 2100 MHz communications band.
During operation, the quality of the wireless link between a portable electronic device and a wireless base station may vary. For example, link quality may decrease when the wireless link is blocked by an obstruction or when the distance between the portable electronic device and wireless base station increases due to device movement. Link quality may increase when the portable electronic device is moved to an unobstructed location or when the distance between the portable electronic device and the base station is reduced.
Because link quality can vary, real-time output power adjustments are typically made during device operation. When wireless link quality is high, radio-frequency signals may be transmitted from the portable electronic device at a relatively low output power. This helps to conserve power and avoids interference with other devices in the network. When wireless link quality is low, radio-frequency transmit powers can be increased to ensure that radio-frequency signals are received satisfactorily at the wireless base station. The wireless base station can send commands to portable devices that inform the devices of necessary power adjustments. These commands, which are sometimes referred to as transmit power control (TPC) commands, can instruct a given device to increase or decrease its radio-frequency output as appropriate.
Successful operation of a portable electronic device generally requires that the device be capable of receiving and processing transmit power control commands from network equipment. When a command is received to increase the output power, control signals are used to adjust internal wireless circuitry to make a corresponding upward adjustment to the power of the transmitted radio-frequency signals. When a command is received that indicates that less output power is needed, the device can respond by instructing the wireless circuitry to decrease the output signal strength.
To ensure that the output powers that are provided are correct, calibration operations may be performed on portable electronic devices. Closed-loop (feedback) control techniques can also be used to ensure accurate output power control. Closed-loop techniques typically rely on power detector measurements that are only accurate near the upper range of commonly used output powers. As a result, it may be desirable to revert to an open-loop control scheme at low powers. Satisfactory transitions between open-loop and closed-loop control regimes can be difficult to achieve, however, because open-loop control schemes are more frequency dependent than closed-loop schemes. This gives rise to a potential for significant output power inaccuracies.
It would therefore be desirable to be able to provide improved radio-frequency output power control in wireless electronic devices.